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Bio 169 Endocrine System Study Guide
Spring Semester 2005

Learning Objectives, Topics, and Key Terms


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Readings: Martini, Ch. 18, pp 605 - 634.

Overall view of hormones:

exocrine vs endocrine secretion (KEY CONCEPT)

endocrine glands are ductless (their cells secrete across the basal membrane -- what is exocrine?)

hormones are chemical signals secreted by endocrine glands

hypophysis (also called, pituitary; intimately related to hypothalamus )

thyroid

parathyroid

pancreas

adrenal

gonads

also from endocrine cells found within most other organs

adipose tissue

brain

gut

kidney

heart

placenta

Comparison of Nervous and Endocrine Systems

nerve cells release chemical (neurotransmitter) locally at synapse

rapid effect (e.g., movement; sensation)

endocrine cells release chemical (hormone) into blood stream

action occurs at some distance (ususally)

slower effect (e.g., lowering of blood sugar)

coordinated action of both systems (i.e., the "neuroendocrine" system)

 

Hormone Action

affect target cells only - HOW?

by binding to specific receptors with high affinity

receptors decrease if a lot of hormone ("down regulation")

receptors increase if a not much hormone ("up regulation")

hormones are inactivated by one of several mechanisms

a circulating enzyme, or chemical modification in liver, or excretion by kidney

Endocrine vs Paracrine vs Autocrine

distant target cell (by circulation) vs local target cell (by diffusion) vs on self

Hormone Chemistry

steroids

derived from cholesterol

e.g., estrogen, cortisol

biogenic amines (amino acid derivatives)

e.g, epinephrine, histamine, thyroid hormone

peptides and proteins

e.g., ADH (a peptide), insulin (a protein)

eicosanoids

e.g., prostaglandins

water-insoluble hormones travel in blood mostly bound to specific transport proteins

Mechanisms of Hormone Action

target cells must have specific receptors for hormone

properties of the target cell determines the response to a hormone

e.g., insulin --> storage of glycogen in hepatocytes but storage of triglycerides in adipocytes

Intracellular receptors

e.g., nuclear receptors turn on genes in DNA --> mRNA --> particular proteins

lipid soluble hormones like thyroid and steroids can reach intracellular receptors

 

Extracellular receptors

water soluble hormones such as epinephrine or insulin can't enter cells

hormone = first messenger -- binds to extracellular receptor

hormone binding to receptor is signaled by a G-protein

intracellular chemical = second messenger

example: cyclic AMP (abreviation: cAMP)

made by adenylate cyclase

cAMP activates protein kinases

kinase phosphorylates target protein -- adds a phosphoryl group

turns on or off the protein (e.g., activates an enzyme or opens a channel)

protein phosphatase removes phosphoryl group to end hormone effect

cAMP destroyed by phosphodiesterase

initiates a cascade or chain reaction leading to amplification of hormone effect

 

 

Control of Hormone Secretion

by nervous system

e.g., epinephrine (by preganglionic fibers of S-ANS)

by another hormone

e.g, pituitary hormones -- TSH --> thyroid

by blood level of metabolite

e.g., parathyroid hormone (low calcium ion in plasma triggers secretion of PTH)

 

Hypothalamus and Hypophysis (Pituitary)

hypophysis located in hollow of the sphenoid bone called "sella turcica"

hypothalamus controls hypophysis (main neuroendocrine link)

anterior lobe of pituitary is glandular (adenohypophysis)

controlled by releasing and/or inhibiting factors made in hypothalamus

posterior lobe of pituitary is neural (neurohypophysis)

made of unmyelinated axons with cell bodies in hypothalamus

 

blood supply to adenohypophysis is via a portal system: the hypothalamohypophyseal portal system

superior hypophyseal arteries form a primary plexus in the hypothalamus

portal veins pass down pituitary stalk

form a secondary plexus in adenohypophysis (anterior lobe)

delivers hypothalamic hormones that control adenohypophyseal cells

hypothalamic neurons secrete hornones that act on adenohypophyseal cells to cause them to

increase or decrease secretion of their hormone (specific to each cell type)

enter primary plexus, travel in portal veins down the stalk, leave secondary plexus

can be releasing hormones (e.g. GHRH) or inhibiting hormones (e.g., GHIH or somatostatin)

five cell types in adenohypophysis

somatotrophs - human growth hormone (hGH; also, somatotropin)

lactotrophs - prolactin - milk production

corticotrophs - adrenocorticotropic hormone (ACTH or corticotropin) - stimulates adrenal cortex to release cortisol, a "glucocorticoid"

thyrotrophs - TSH - stimulates thyroid gland

gonadotrophs - LH and FSH - act on gonads

 

negative feedback regulation - thyroid hormone as example

plasma level of T3 rises -->

more T3 bound to T3 receptors in the nuclei in TRH and TSH cells in hypothalamus and ant. pit. -->

more inhibition of specific DNA synthesis coding for these -->

less secretion of TRH and TSH -->

plasma level of T3 falls --> less inhibition of specific DNA synthesis in those cells --> et cetera

 

 

Growth Hormone (also called somatotropin)

actions: increase amino acid uptake and protein synthesis; release triglycerides from adipocytes (lipolysis); release glucose from liver

promote skeletal growth, growth of other organs

effects may be direct or by release of somatomedins (growth factors) by liver

pathology: too little -> pituitary dwarfism; too much -> pituitary giantism or acromegaly

Posterior Pituitary (Neurohophysis)

hormones are first synthesized in hypothalamic neurons (peptides)

released in neurohypohysis at axon terminals when action potentials pass down axons

antidiuretic hormone (ADH) - promotes water retention in kidney

pathology: diabetes insipidus from insufficiency (e.g., after brain injury)

oxytocin

contracts uterine smooth muscle at parturition

‘pit drip’ (IV infusion of Pitocin®) used to induce labor

contracts myoepithelial cells of mammary glands to force milk out

neuroendocrine reflex (suck nipple --> oxytocin secretion --> "letdown")

Thyroid Gland

Thyroid Hormone (TH is a mixture of two separate substances:T3 and T4)

Synthesis

iodide trapping by powerful iodide pump

thyrocytes secrete "colloid" into follicle lumen (colloid =thyroglobulin + peroxidase + iodide

T3 and T4 form on thyroglobulin.

TSH activates pinocytosis and digestion of thyroglobulin in lysosomes.

--> T3 and T4 released in blood.

regulation: negative feedback onto hypothalamus and thyrotrophs

actions: increase heat production by increasing number of Na-K pumps which turn ATP into heat

promote growth and development (especially of nervous system); up regulate adrenergic receptors --> fast heart rate if too much TH

pathology: hypothyroidism - too little (from insufficient dietary iodide, for example) -->goiter

cretinism

hyperthyroidism - too much (from Graves’ disease for example) --> thyrotoxicosis

can destroy bad thyroid with radioactive iodine (radiothyroidectomy)

Calcitonin - secreted by C cells (parafollicular cells). Lowers plasma Ca++ by activating osteoblasts. C cells respond directly to eleveated calcium ion levels in plasma.

Parathyroid Gland

2 pairs on back of thyroid - very small

secrete parathyroid hormone (a protein) from chief cells

secretion controlled by plasma calcium ion levels

if Ca2+ falls, secretion of PTH goes up

action: increase osteoclastic activity in bone to release calcium, increase calcium retention in kidney, promote calcium uptake from gut

pathology: insufficiency leads to hypocalcemic tetany

Endocrine Pancreas

islets of Langerhans, small balls of endocrine cells amongst exocrine tissue

beta cells secrete insulin

alpha cells secrete glucagon

actions of insulin: lower plasma glucose levels by increasing glucose entry in cells with

insulin receptors. main target cells: liver (glycogen formation), muscle (ditto), adipocytes (triglyceride synthesis)

actions of glucagon: raise plasma glucose, from liver, by promoting glycogenolysis

control of secretion: alpha and beta cells sense plasma glucose directly

pathology: insufficient insulin or insulin receptors leads to diabetes mellitus

high blood glucose, glucose in urine, polyuria, polydipsia, signs of cellular starvation, damage to endothelium by glucose reacting with tissue proteins (glycosylation - measure hemoglobin)

Adrenal Gland (Cortex and Medulla)

anatomy: on top of kidney; blood supply is arterial plexus; connective tissue capsule

cortex has three cell layers: all secrete steroids; synthesized from cholesterol

zona glomerulosa (aldosterone, a mineralocorticoid)

zona fasciculata (cortisol, a glucocorticoid)

zona reticularis (androgens)

actions: aldosterone conserves sodium and promotes potassium loss in kidney

cortisol increases blood glucose from liver by promoting gluconeogenesis

androgens promote blood production, support libido (important in woman)

control of secretion: aldosterone - serum K+ has directly stimulates release; also, by angiotensin II

cortisol - by ACTH (negative feedback of cortisol on corticotrophs and hypothalamic CRF cells)

medulla - postganglionic sympathetic neurons is embryonic source

"cell bodies without axons"

epinephrine (also called adrenaline) is main hormone

actions: increase heart rate and power, dilate bronchial tree, mobilize sugar and fatty acids from liver and adipocytes - cAMP based control system

control of secretion: impulses in preganglionic sympathetic axons trigger release

Endocrine System Study Aid

Complete this table to organize your study of endocrines. Bring to exam!

Hormone

Cellular Source

Target Cell(s)

Action(s)

Regulation of Secretion

hGH

TSH

ACTH

Prolactin

FSH

LH

ADH

Oxytocin

T3, T4

Calcitonin

PTH

Aldosterone

Cortisol

Adr. Androgens

Epinephrine

Glucagon

Insulin


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Last updated 12 December 2005